1. Field of the Invention
The present invention relates to high density memory devices based on phase change based memory materials, including chalcogenide based materials and on other programmable resistive materials, and methods for operating such devices.
2. Description of Related Art
Phase change based memory materials, like chalcogenide based materials and similar materials, can be caused to change phase between an amorphous state and a crystalline state by application of electrical current at levels suitable for implementation in integrated circuits. The generally amorphous state is characterized by higher electrical resistivity than the generally crystalline state, which can be readily sensed to indicate data. These properties have generated interest in using programmable resistive material to form nonvolatile memory circuits, which can be read and written with random access.
The change from the amorphous to the crystalline state is generally a lower current operation. The change from crystalline to amorphous, referred to as reset herein, is generally a higher current operation, which includes a short high current density pulse to melt or breakdown the crystalline structure, after which the phase change material cools quickly, quenching the phase change process and allowing at least a portion of the phase change material to stabilize in the amorphous state.
In phase change memory, data is stored by causing transitions in an active region of the phase change material between amorphous and crystalline states. FIG. 1 is a graph of memory cells having one of two states (storing a single bit of data), a low resistance set (programmed) state 100 and a high resistance reset (erased) state 102 each having non-overlapping resistance ranges.
The difference between the highest resistance R1 of the low resistance set state 100 and the lowest resistance R2 of the high resistance reset state 102 defines a read margin 101 used to distinguish cells in the set state 100 from those in the reset state 102. The data stored in a memory cell can be determined by determining whether the memory cell has a resistance corresponding to the low resistance state 100 or to the high resistance state 102, for example by measuring whether the resistance of the memory cell is above or below a threshold resistance value RSA 103 within the read margin 101.
In order to reliably distinguish between the reset state 102 and the set state 100, it is important to maintain a relatively large read margin 101. However, it has been observed that some phase change memory cells in the reset state 102 will experience an erratic “tailing bit” effect in which the resistance of the memory cell decreases over time to below the threshold resistance value RSA 103, resulting in data retention problems and bit errors for those memory cells. Error Correction Coding (ECC) may be used to address the tailing bit issues, but may result in penalties regarding write efficiency, read speed, and chip size.
It is desirable therefore to provide a memory cell structure and methods for operating such structures which address these data retention issues without ECC and result in improved data storage performance.